the major difference between the pedals is in the way the float works. the time pedals have a return spring that pushes your foot back to the neutral position while the speedplays have a free float. which is better kind of comes down to personal preference and what your own neutral position is. personally, i can't stand free float...i have never felt comfortable sprinting with free float pedals but feel much safer with the times. i too have never had an accidental release with time pedals. however, if your toes naturally point in, you might have problems with the return spring in the times.
also, the times have lateral float in addition to the rotational float.
one big thing to keep in mind between these two pedals as well is that the listed weights are misleading. the weight often given for the speedplays is for the pedal alone and ignores the fairly significant weight of the cleat, which is where the engagement mechanism is on the speedplays. while the pedal and cleat combo is still a little lighter for the speedplays, it is nowhere as significant as it seems when you read the listed weights in a catalogue.
i think that the best advice has been given, however, and that would be to try out each pedal type if you can.
good luck!